This application claims priority of Taiwanese application No. 091111960, filed on Jun. 4, 2002.
1. Field of the Invention
This invention relates to a heterojunction bipolar transistor, more particularly to a heterojunction bipolar transistor with zero conduction band discontinuity.
2. Description of the Related Art
Recent developments in the field of heterojunction bipolar transistors (HBT) focused primarily on high-speed, lower power consumption, low turn-on voltage, low compensation voltage, high breakdown voltage, high linearity, and high frequency analog and digital circuit applications. FIG. 1 illustrates a conventional GaAs-based HBT, such as AlGaAs/GaAs or InGaP/GaAs HBT, that normally includes a semiconductor (otherwise known as semi-insulating material) GaAs substrate layer 11, a GaAs buffer layer 12 formed on the substrate layer 11, a GaAs collector layer 13 formed on the buffer layer 12, a GaAs base layer 14 formed on the collector layer 13, an AlGaAs or InGaP emitter layer 15 formed on the base layer 14, and an emitter cap layer 16 formed on the emitter layer 15. The conventional HBT is advantageous in that (1) the use of wide energy band AlGaAs or InGaP for making the emitter layer 15 can effectively reduce injection of minority carriers from the base layer 14 into the emitter layer 15, which, in turn, can increase the injection efficiency of the emitter layer 15, (2) that a relatively high doping concentration in the base layer 14 is permitted, which can significantly reduce the resistance of the base layer 14 and which can considerably increase punch-through voltage, and (3) that a lower doping concentration in the emitter layer 15 is permitted, which can effectively reduce capacitance between the emitter layer 15 and the base layer 14, which, in turn, is advantageous for high frequency response applications. With the advantages as mentioned above, the conventional HBT is particularly suitable for use in microwave power amplifier applications and high frequency voltage controlled oscillator applications. In addition, as compared to the InP-based HBT, the GaAs-based HBT has higher production yield and reliability and involves lower manufacturing costs.
However, the conventional GaAs-based HBT is disadvantageous in that, when under a biased condition, an undesired potential spike occurs due to the presence of conduction band discontinuities (xcex94Ec) at junctions 18, 17 (see FIG. 2) between the emitter layer 15 and the base layer 14 and between the emitter layer 15 and the emitter cap layer 16 upon actuation of the HBT, which can deteriorate the performance of the HBT, such as a decrease in the emission efficiency and in the collector current, and an increase in the recombination current, in the base-to-emitter turn-on voltage, and in the collector-to-emitter compensation voltage. The aforesaid undesired effects are particularly severe under low working current conditions.
Therefore, the object of the present invention is to provide a heterojunction bipolar transistor that is capable of overcoming the aforesaid drawbacks of the prior art.
According to the present invention, there is provided a heterojunction bipolar transistor that comprises: a collector; a base layer formed on the collector; a first transition layer formed on the base layer; an emitter layer formed on the first transition layer; a second transition layer formed on the emitter layer; and an emitter cap layer formed on the second transition layer. Each of the first and second transition layers is formed of a composition that contains an element. The element of the composition of the first transition layer varies decreasingly in mole fraction from one side of the first transition layer, which is adjacent to the emitter layer, to an opposite side of the first transition layer, which is adjacent to the base layer. The element of the composition of the second transition layer varies decreasingly in mole fraction from one side of the second transition layer, which is adjacent to the emitter layer, to an opposite side of the second transition layer, which is adjacent to the emitter cap layer, so as to form a continuous conduction band from the base layer, through the first transition layer, the emitter layer and the second transition layer, to the emitter cap layer.